The invention relates to a device and a method for the optical detection of substances contained in the exhaust gases of chemical processes where said substances are formed, released and transported as a component of an exhaust gas flow from a reaction or vacuum chamber in various chemical processes.
With conventional solutions it is normal practice to extract individual samples successively from the exhaust gas flow and to perform a corresponding detection in more or less large time intervals for the individual samples in each case. Subsequently, no time-proximal intervention in the individual process can take place.
The extraction of a partial flow from the overall exhaust gas flow is also disadvantageous. Here, the overall exhaust gas volume subjectable to detection is reduced and, in particular, the absolute volume of the substances to be detected is reduced accordingly, whose component can already be very small in any event. As a result, detection errors can occur that can be of such an extent that traces of even essential substances in the exhaust gas cannot be recognised or can be recognised with a great timing delay only. Due to the timing delay, unstable compounds (so-called intermediates) cannot be detected or can be detected with limitations only. The intermediates are often process-domineering, so that their correct verification can be particularly important for a process control.
With a continual detection, also on the optical route in the comprehensive exhaust gas flow as described for example in EP 0 768 525 B 1, solid particles and/or aggressive gases, which are frequently contained in the exhaust gas flow in a relative large volume therein, lead to considerable disadvantages.
Solid particles disturb not only the optical detection as such because, as a rule, they are not transparent for the individual electromagnetic radiation or are light-scattering and on the route through an optic measuring cell they frequently and optically block or obstruct the beam path of the radiation, but they are also a problem as a result of the frequent adhesion to the wall or also to windows within measuring cells. This causes a layer formation which leads to a reduction of the cross-section and this again leads to an increase of the flow velocity and to turbulence formation of the exhaust gas flow. The window scale deposit reduces the transmissivity and, subsequently, a cleaning and/or a replacement of windows is required within short time intervals.
As a result, a cleaning of measuring cells in more or less short time intervals is required which often do not coincide with cleaning or maintenance cycles of the process technology as required in any event. Downtime results in this way or at least a redundant additional measuring system is required. Both are undesirable from the aspect of costs.